Electrical network



?atented er.. 15, 1.935

4 UNITED STATES PATENTA oFFlcE ELECTRICAL' NE'rwonx Irving wom,Merchantviue, N. J., signor te Radio Corporation of America, a'corporation of Delaware Application October 29, 1932, Serial No. 640,249

9Claims.

monies and combination tones in the reproduction of sound and limits thepower available from l most tubes.

Non-linear distortion in vacuum tube circuits is due, principally, tothree causes:

1. Variation in the plate resistance--the resistance normally assuminglarge values for small plate potentials and decreasing rapidly as theplate potential is decreased.

2. Variable amplication factor. For three element tubes, this is notserious, but in tubes of four or more elements the change inamplification factor may be important.

3. .Variable low input impedance-in certain types of amplifiers wherevthe grid is allowed to swing positive, the input impedance of the gridcathode circuit varies with the amplitude of the impressed signals. Thisis a very serious cause of distortion.

It is, accordingly, an object of my invention to provide an electricalnetwork whereby a thermionic tube may be more efliciently utilized foramplification-or other purposes.

Another object of my invention is to provide an v means whereby therange of available swing of a Y tube may be greatly extended so thattubes having smaller plate and filament consumption than have heretoforebeen used may be made to deliver adequate power.

A still further, and more specific, object of my invention is to providean electrical network that shall enable a thermionic tubek incorporatedtherein to-give greater undistorted output than heretofore.

00 The foregoing objects and 'others ancillary thereto I attain, in apreferred embodiment of my invention, by causing the output current of athermionic tube totraverse an electrical network that does not obey`Ohms law. In other words,

55 in a network constructed and arranged according to my invention, Iinterpose one of more elements having such characteristicsthat the rateof change of potential thereacross is not linearly related to the rateof change of current therethrough.

A material .that I have found very satisfactory for the purpose iscommercially known as Thyrite, and is described in the McEachron UnitedStates Patent No. 1,822,742.

Thyrite, as described in the patent referred to, 10 is a materialsomewhat similar to dry process porcelain throughout the mass of whichappear A minute particles of conducting material. It is substantially aninsulator at low potentials and becomes a continuously better conductoras the 15 current through it is increased, even in some instances in theratio of 10,000,000 to one. Thyrite can be constructed to have a lawconnecting resistance and current through it of the following form oversubstantial current ranges: 20

C R n Eea where the exponent can 1tary from il, which means constant Ras in an ordinary wire resistance, to 35 somewhat more than 2.

In terms of voltage, current is proportional to where the exponent canbe made to vary from 1 to about 3.5.

The material is usually utilized in the form of discs, each face ofwhich is provided with a sprayed metallic coating for contact purposes,but 45 the shape is not a'material factor. It should be noted, however,that the resistance of Thyrite varies directly with its thickness, butnotinversely with area, as does that of resistor materials, such ascarbon, or metals which have heretofore been utilized in couplingnetworks.

The novel features that I consider characteristic of my invention areset forth with particularity in the appended claims. The inventionitself, however, both as to its organization and its method ofoperation, together with additional objects and advantages thereof, willbest be understood from the followingdescription of a specificembodiment, vwhen read in connecg tion with the accompanying drawing,wherein equivalent elements are similarly designated, and in whichFig..1 shows a family of plate current-plate potential curves for atypical thermionic tube of .the triode type;

Fig. 2 is a curve illustrating the current-potential characteristics oia preferred material, such as Thyrite, and

Figs. 3 to I are views of varlous'thermionic ll tube utilizationnetworks embodying my invention. t Fig. 1 shows a family of platecurrent plate potential curves for a vacuum tube of the type to whichthe plate resistance changes, but the N amplification factor remainsfairly constant. The usual performance of the tube is determined bydrawing the straight line AB giving the relation between plate currentand plate potential for a fixed resistance.

with the different grid potential lines Ec determine the gridpotential-plate potential characteristics, the potential across the tubebeing given by the voltage shown in the abscissae scale and that acrossthe external resistance by the hori- 80 zontal distance between the lineBC and the points of intersection. Ii' the series of grid potentiallines are drawn for equal increments of grid voltage, then, in order toobtain a linear relation between voltage developed across the u externalresistance and grid voltage, the distance between each one of thesucceeding intersections and the line BC must differ by a constantamount from the preceding one. Reference to Fig. 1 shows this is nottrue, and referen to a number of other tube characteristics will showthe same effect. Y

It will, further, be noted that the horizontal distance betweensuccessive intersections on the left end of the curve is very nearly thesame.

Taking this distance, a new line AD may be drawn such that all points ofintersection differ by a constant amount. If a resistance existed suchthat its E-I curve were of the same shape as that shown by the curve AD,where the current is measured as usual and the voltage is measured fromsome point of intersection of AD produced with the O current axis,non-linear distortion would be eliminated in the region 55 underconsideration. vSuch an E--I curve can be obtained by Thyrite circuits,as will be shown later.

The series of characteristics which have been shown in Fig. 1 arerepresentative of the three 60. element tube. Tubes with more elementsshow a changing amplification factor aswell as -a change in plateresistance. A series of curves similar to those shown in Fig. 1 may bedrawn, however, and the same procedure can be fol- 65 lowed indetermining the E-I curve required in the external circuit.

If, in addition to the above types of distortion, the third type, viz.,variable low gridcathode resistance is in effect, a somewhat dif- 70ferent family of curves must be drawn to obtain the E-I relationrequired for correction. As long as the inputV impedance of the tubeunder Aconsideration is high or constant, compared to the feedingcircuit, E@ is equivalent or, at least,

75 proportional to the voltage' El developed in the importance.

Thel points of intersection A output circuit of the preceding tube whenit is feeding into, a very large impedance.

A' new series of curves might, therefore, be constructed using El inplaceof E which would be exactly the same as the 'series in Pig. 1. 5

If, however, the input impedance of the under consideration becomes lowcomparedtor` the preceding tube output circuit, this relation vno longerkholds and the El family will differ from the Ee family, particularlyfor positive values 10 of Ee where the drop of impedance becomes of Ifthe output impedance of the tube and the impedance of the feedingcircuit are known, vthe values of Es corresponding to any value of Ercanbe calculated and the El 15 family of plate current plate voltage curvescan. be drawn. The effect will be to crowd the El curves closer togetherfor positive values. Using the same procedure as in' the precedingcases, a curve EAD can be drawnao which represents the E-I relationwhich must hold in the plate resistanceto obtain linearoutput. Thiscurve diers from that required for the first two types of distortion inthat the resistance must increase both for high and low 25 values'of thecurrent. A circuit employing Thyrite can be constructed to do this, aswill be shown.

The non-linear current-voltage characteristics of 'I'hyrite areexemplified by the curve shown 30' in Fig. 2, from which it will benoted that, although the potential drop across the material increaseswith increase in current, the rate of and being coupled through themedium of a 45 portion of Thyrite 1. A resistance coupled amplifier ofthe type shown, wherein the coupling resistor is of the usual type, isincapable of distortionless amplification if the grid potentials aresuch as to cause the tube to work over portions of its characteristiccurve which are nonlinear. If, however, Thyrite having the properconstants is utilized for the coupling resistor, the value of theresistance decreases with increase in plate current and decreases duringdecrease in plate current with reference to its steady or DC value, thuscompensating the nonlinearity `of the tube characteristic.

Under certain conditions, it may be desirable to utilize that portion ofthe I -E curve of Thyrite lying to the leftvof the origin, as shown inFig. 2. In such event, the Thyrite is initially biased, as to thepotentials indicated by the lines BB or CC, by the application theretoof biasing potential from any convenient source II.

By reason of the fact that Thyrite shows a symmetrical characteristicfor both negative and positive currents, it may be used in connectionwith elimination or application of predetermined amounts of directcurrent to obtain a wide variation in resistance-Voltagecharacteristics. An in'- stance of this is shown in Figs. 4 and 5, whichrepresent satisfactory networks. The Thyrite marked 1" is used in thenormal fashion, with all the plate current flowing through it, the 'Ihyrite 75" marked 9 has a large condenser in series with it and isshunted by a resistance which is higher than the values assumed by theThyrite, or which may have a value given to it which will limit theextent to which the resistance of this circuit can increase. When noalternating current is flowing through the vacuum tube, there is acertain-drop across the external circuit and a certain voltage acrossit. 'I'he resistance voltage curve of Thyrite 'l is determined by thedirect current, while that of Thyrite 9 is determined by the alternatingcurrent. As a function of the direct current when an alternating currentis applied to the input of the tube, the resistance of Thyrite 9decreases for both an increase or decrease of direct current in theplate circuit.

As an additional modification, a certain amount of direct current mightbe applied to Thyrite 9 by means of an external supply source Il, or bysome shunt connection from the regular plate supply, and additionalmodifications in the total resistance-potential characteristic could beobtained. It is evident that this type of connection may be used toobtain a wide variety of characteristics.

It is also feasible to employ a network constituted by a plurality ofThyrite resistors 'l and Il in series, as shown in Fig. 6 of thedrawing, the several resistors being respectively biased in oppositedirections by a common source I5.

The arrangement illustrated in Fig. '1 may also be employed, a Thyriteresistor I1 being connected in shunt to the secondary winding. of acoupling transformer I9. If desirable, the Thyrite may be polarized by asource 2l, an additional current limiting resistor 23 being interposedin series therewith, if desired.

Resistance coupled circuits have been chosen for illustration because ofthe simple theory involved. Since, however, the use of a transformermerely reflects the resistance characteristics of the secondary circuitback to the primary side, it is evident that any of the results whichhave been described for resistance coupled circuits can be obtained bythe use of a transformer and Thyrite in the secondary circuit and aproper biasing battery.

It will be apparent from the foregoing that a thermionic tubeutilization network embodying my invention has many' advantages. Muchgreater undistorted power can be obtained from a thermionic tube thanhas heretofore been possible and, in fact, by properly choosing the biaspotentials applied to my improved coupling resistor and the manner inwhich one or more resistor elements are connected in the output circuitof the tube, substantially any irregularities in the tubecharacteristics can be compensated.

Furthermore, my invention is not limited to the use of Thyrite, or anequivalent material as a coupling element between thermionic tubes or asmeans for compensating undesirable tube characteristics, since manymodifications will at once be apparent to those skilled in the art towhich it pertains. My invention, therefore, is not to be limited by thespecific exemplication chosen for purposes of illustration, but only bythe prior art and by the spirit of the appended claims.

I claim as my invention:

l. A thermionic tube utilization network including at least one elementwhich has a hyperbolic resistance-ampere characteristic and means forsubjecting said element to a biasing potential,

such as to enable the network to operate with a linear responsecharacteristic.

2. In combination, a .thermionic tube having non-linear characteristics,an output circuit connected thereto and means having a hyperbolic re- 5sistance-ampere characteristic included in the output circuit thereoffor introducing distortion of sense opposite that caused by thenon-linear characteristics of the tube.

3. A thermionic tube having a plurality of electrodes and a networkincluding at least one resistance element connected between two of theelectrodes of said tube, the value of said resistance element beingproportional to CI- over a substantial range, where C represents aconstant which determines the resistance when current is unity and a. isanother constant depending upon the physical structure of the resistanceelement, and the structure of the tube being such that its normallynon-linear characteristic, when operated in the said network exclusiveof said resistance element, assumes a linear characteristic by virtue ofthe introduction of said resistance element into said network.

4. An electron tube network comprising at least one resistance elementthe resistance value of which decreases more and more rapidly with thepassage therethrough of given increments of current, an electron tubehaving a non-,linear relation between potential variations occurring inthe output circuit and simultaneous variations occurring in the inputcircuit, and means including a source of biasing potentials for causingsaid resistance element to compensate for the non-linear characteristicof said tube.

5. A network including an electron tube and a plurality of resistanceelements as defined in claim 4, reactive means inter-connecting saidelements in series and means including an ohmic resistor shunting saidreactive means and one of said resistance elements for enabling saidelectron tube tgtifunction as though it had a linear character- 6. Anetwork including an electron tube and a plurality of resistance2elements as dened in claim 45 4, reactive means interconnecting saidelements in series and means including an ohmic resistorshunting saidreactive means and one of said resistive elements, also a source ofbiasing potential in parallel with one of said resistance elements forenabling said electron tube to function as though it had a linearcharacteristic.

7. A network including an electron tube, and a plurality of resistanceelements as defined in claim 4, said elements being connected in seriesand being supplied with potential for biasing the same in oppositedirections.

8. A network including an electron tube, a coupling transformer and, inshunt with the secondary winding of said transformer, a resistanceelement as defined in claim 4, the elements in combination havingindividually such values as to cause the network to operate with alinear response characteristic.

9. 'I'he method of extending the range of avail- 65

